The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the in vivo measurement of temperature changes using NMR imaging techniques.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR-guided interventional procedures employ the MRI system to produce real-time images which enable the procedure to be monitored. Such procedures, include MR-guided biopsies, hyperthermia, cryoablation, and ablation using laser, radiofrequency, and focused ultrasound. A critical part of such MR-guided ablation procedures is the ability to monitor spatially localized changes in temperature using heat-sensitive MR pulse sequences. Four different NMR properties of tissues have shown potential as parameters sensitive to temperature changes. These are the spin-lattice relaxation time T1, the molecular diffusion coefficient D, and the water proton chemical shift (PCS). The PCS method is based on the dependence of the water proton resonance frequency on temperature. Using this phenomenon, the phase of a gradient-echo image can be used to measure temperature as described, for example, in U.S. Pat. Nos. 5,307,812; 5,323,779; 5,327,884 and 5,711,300.
It has been shown that to achieve the optimal signal-to-noise ratio (SNR) in a temperature-sensitive phase image, the echo time (TE) of the pulse sequence used to acquire the NMR data should be equal to the spin-spin relaxation time constant T2* of the imaged subject matter. The T2* constant of tissues is typically on the order of 40 ms. As a result, a conventional scan can be quite long in duration if the TR period of the pulse sequence is set long enough to accommodate a 40 ms TE. In a 256xc3x97256 voyel 2D acquisition using pulse sequence with a TR=50 ms, for example, 13 seconds is required to acquire a complete 2D image data set. When a 3D temperature map is to be produced, the acquisition time becomes much longer and is not very useful in real-time imaging applications.
The present invention is a method for producing a temperature map which indicates the temperature of in vivo tissues or in vitro temperature calibration phantoms. More particularly, the present invention is a method in which an NMR contrast agent which alters the spin lattice relaxation time (T1) of spins in the subject to be measured is applied to the subject, an NMR pulse sequence is performed with an MRI system to acquire an NMR data set from which an image may be reconstructed, and a phase image which indicates temperature is reconstructed from the NMR data. The NMR pulse sequence is continuously repeated to update the acquired NMR data such that temperature maps may be produced in real time to indicate temperature changes occurring during a medical procedure or the like. The temporal rate at which updated temperature maps are produced can be further increased by updating the NMR data set with central k-space samples at a higher rate than peripheral k-space samples are acquired.
A general object of the invention is to increase the temporal rate at which NMR temperature maps can be produced without reducing their SNR. It has been discovered that NMR contrast agents, such as Gd DPTA, enable the TE/TR period of a PCS sensitive NMR pulse sequence to be substantially reduced without diminishing the SNR of the resulting temperature map. This results in a shorter scan time and a consequent higher temperature map temporal rate. Importantly, it has been discovered that the temperature sensitivity of the NMR pulse sequence is not significantly altered by the contrast agent and the temperature measurement is not significantly affected by variations in contrast agent concentration.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.